Present trends towards smaller and cheaper wireless communication devices have resulted in the development of on-chip replacements for bulky intermediate frequency (IF) surface acoustic wave (SAW) filters, which had been widely used in the past. However, unlike SAW filters, on-chip filters cannot be fabricated precisely because of the inherent random variations in the fabrication process for integrated circuits. Because of this, on-chip filters generally have a tuning mechanism that tunes their center frequencies and quality factors to assure their performance over all operating conditions.
The quality factor (Q) is inversely proportional to the bandwidth of the filter. Thus, if Q is more than the desired value, the bandwidth is less than the desired bandwidth. In this situation, the filter does not select the whole bandwidth and the system loses information. Similarly, if Q is less than the desired value, the bandwidth is too large. In this situation, the filter picks up noise along with the signals, making signal processing difficult. Therefore, the Q value of the filter needs to be accurately tuned in order to obtain optimal performance from the communication system.
Conventional methods for tuning Q include magnitude locked loops and envelope detection. Magnitude locked loops use the fact that the midband gain of the filter is proportional to Q. This method tunes Q indirectly by tuning the filter's gain. In doing this, the method employs a magnitude locked loop around the filter and compares the filter output signal strength with a reference amplifier output, tuning the filter gain with a resulting error signal. However, the frequency of operation is limited by the reference amplifier gain-bandwidth, and a reference amplifier with a gain proportional to Q at the filter center frequency is required. Thus, although this method works well for relatively low-Q, low-frequency bandpass filters, it fails to provide accurate Q tuning for filters with a Q value of about 100 or higher and with center frequencies of about 10 MHz or higher.
The envelope detection method tunes Q indirectly by adjusting the filter's transient characteristics associated with the filter step response. In doing this, the method compares the filter output transient envelope with a reference envelope, tuning the filter output envelope with a resulting error signal. Tuning the filter output envelope in this manner tunes the filter transient time constant, which is proportional to Q. However, the frequency of operation is limited by the center frequency signal component in the step function, and the envelope detection method requires a very fast peak detector for high frequencies. Thus, although this method works well for relatively low-Q, low-frequency bandpass filters, it fails to provide accurate Q tuning for filters with a Q value of about 100 or higher and with center frequencies of a few MHz or higher.
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